This disclosure relates to an OLED display device, a circuit therein, and a method of manufacturing the OLED display device.
The organic light-emitting diode (OLED) element is a current-driven self-light-emitting element and therefore, eliminates the necessity of backlight. In addition to this, the OLED display element has advantages for achievement of low power consumption, wide viewing angle, and high contrast ratio; it is expected to contribute to development of flat panel display devices.
An active-matrix (AM) OLED display device includes selecting transistors for selecting pixels and driving transistors for supplying electric current to the pixels. The transistors in an OLED display device are thin film transistors (TFTs); particularly, low-temperature polysilicon (LTPS) TFTs are commonly used. Furthermore, a typical OLED display device includes a compensation circuit for compensating for the differences and variation in threshold voltage of the driving transistors.
TFTs having existing structures have hysteresis characteristics. Because of the hysteresis characteristics, the drain current flows differently in raising and lowering the gate potential. The hysteresis characteristics of the driving transistors cause a phenomenon called image retention. For example, when an OLED display device is controlled to change the display to a white picture after displaying a black picture for a while, the screen does not become white upon the operation to change but takes several frames to display the white picture.
When no current flows in a driving TFT for a long time, the hysteresis of the driving TFT is initialized. If a Vgs bias to display a white picture determined based on the initialized hysteresis is applied, the current decreases instantly because of the hysteresis, so that the intended brightness for the white picture is not attained.
Meanwhile, in terms of manufacturing an OLED display device, efficient processes are desired. An OLED display device has a multi-layer structure including pixels with organic light-emitting elements and pixel circuits for controlling the intensity of light to be emitted from the pixels. Manufacturing an OLED display device repeats patterning using a mask having a different opening pattern to form the pixel circuits and organic light-emitting elements on a substrate.